Additive for copper graphite brush



United States Patent ADDITIVE FOR COPPER GRAPHITE BRUSH DimiterRamadanolf, Berea, Ohio, assignor to Union Carbide Corporation, acorporation of New York No Drawing. Application March 8, 1955 Serial No.493,069

2 Claims. (Cl. 252-503) This invention relates to brushes for the supplyof electric current, and more particularly concerns long wearingcommutator brushes.

Metal graphite brushes such as the conventional copper-graphite brushescontaining about 30 percent to 75 percent copper, and 25 percent to 70percent graphite have several desirable properties. At low currentdensities and rubbing speeds, that is, about 60 amperes per square inchand 2500 feet per minute, respectively, the copper graphite brushgenerally operates satisfactorily with a low contact voltage drop and alow temperature. When used under these conditions, the copper-graphitebrush is free from dusting and can render service for a relatively longtime. But when more stringent conditions are imposed on the. brush, theoperating life is considerably impaired. Increasing either the currentdensity or the rubbing speed can cause a rapid increase in brush wear.For example, at a rubbing speed of approximately 5000 feet per minuteand current density of 120 amperes per square inch, the performance ofthe conventional brush is hampered by the elfects of ring wear, highcoefficient of friction and dusting. The brush is also sensitive tocommutator or ring eccentricity. Often the combination of these factorswill intensify sparking, burn portions of the ring surface, and evencause the release of small streamers at the trailing edges.

It is, therefore, an important object of the present invention toprovide an improved metal graphite brush composition which is longwearing and suitable for heavy duty purposes.

According to the present invention it has been found that the life of aconventional metal graphite sliding contacting brush can be greatlylengthened and contact conditions materially improved if the brushcomposition is made from certain materials.

A brush embodying the principles of my invention may be readily formedby introducing as additive material into a conventional metal graphitebrush composition of approximately 30 percent to 75 percent copper and25 percent to 70 percent graphite, small quantities of lead and boronnitride. present in the brush composition in amounts between 1.1 percentand 15 percent by weight of the copper graphite mixture.

Highly satisfactory results have been obtained in the practice of theinvention when the percentage composition of the above additives areWithin the composition range listed below in Table I.

These combined additives may be 2,844,544 Patented July 22, 1958 ICCTABLE I Composition, Percent Additive Broad Preferred Range, Range,percent percent {Lead 1.0 to 10 4 to 6 Boron Nitride 0.1 to 5 0.5 to 1.5

An example of the practice of the invention is as follows:

EXAMPLE I The composition of a copper graphite mixture containingapproximately percent copper, 50 percent grahite and suitable bindermaterial was modified by adding to it about 5 percent lead powder and 1percent boron nitride powder. The said ingredients were then well mixedand molded at a pressure of 15 tons per square inch to a block ofsuitable size for making a commutator brush. The block was then baked at900 C. by suitable heating means for a period sufiicient to impart knownand inherent brush characteristics.

Table II sets forth the' conditions and results of a number of typicaltests wherein the brush was tested at sea level conditions. In thistable the brushes were subjected to current densities of amperes persquare inch and amperes per square inch. The procedure used for testingthe brushes consisted in applying the brush to a machine having anoperating speed of about 6500 revolutions per minute, giving a rubbingspeed of 5100 feet per minute. A brush spring pressure between 7.1 p. s.i. and 7.3 p. s. i. was used. The current flow through the brush wasadjusted to between 20 and 40 amperes at .a current density between 60amperes per square inch and 120 amperes per square inch. Tests wereconducted in room air for a total of 20 hours or more. During that timereadings were taken of contact drop voltage, brush temperature, andcoeflicient of friction. At the end of the tests brush length wasmeasured and brush life in hours per inch of wear was calculated.

TABLE H Brush composition A=Oopper, 50 parts Graphite, 50 parts B=Oopper, 50 parts Graphite, 50 parts Lead, 5 percent Boron nitride, 1percent 3 brushes of the copper graphite type, better performance can beobtained if the brushes contain boron nitride and lead within theaforesaid composition range.

The improved performance manifests itself in longer brush life, smallerring wear, low coefficient of friction, low operating temperatures, andgreater current load capacity.

It will be understood that modifications and variations may be efiectedwithout departing from the novel concents and scope of the presentinvention.

What is claimed is:

1. An electrical contact brush consisting of about 30% to 75 copper, 25%to 70% graphite, 0.1% to 5% boron nitride and 1% to 10% lead, said boronnitride and lead rendering said brush substantially resistant to dustingconditions and imparting longer wear life thereto.

References Cited in the file of this patent UNITED STATES PATENTS1,121,960 Whitney Dec. 22, 1914 2,156,803 Cooper May 2, 1939 FOREIGNPATENTS 712,188 Great Britain July 21, 1954

1. AN ELECTRICAL CONTACT BRUSH CONSISTING OF ABOUT 30% TO 75% COPPER,25% TO 70% GRAPHITE,2.1% TO 5% BORON NITRIDE AND 1% TO 10% LEAD, SAIDBORON NITRIDE AND LEAD RENDERING SAID BRUSH SUBSTANTIALLY RESISTANT TODUSTING CONDITIONS AND IMPARTING LONGER WEAR LIFE THERETO.